1. Field of the Invention
The present invention generally relates to a tubing drain valve suitable for use in a borehole and a method for operating the disclosed valve. The present invention is useful in a production string where it provides a convenient apparatus and method for draining production fluid from the string in order to facilitate routine activities, e.g., removal of the downhole pump, and is particularly useful in strings used in the production of heavy, sand-bearing oils. More particularly, the present invention relates to a tubing drain valve actuated to its open position by causing a plurality of dogs carried by a sleeve disposed about the tubular member bearing the valve to engage temporarily the casing of the well bore to actuate the valve passage to the open position.
2. Description of the Background
It is often desirable to have a drain valve located in the wall of a borehole tubing string. It is particularly desirable to have a tubing drain valve located in a production string for a variety of reasons. For example, a tubing drain valve would be useful in a production string to facilitate the draining of the column of production fluid in the string prior to pulling the string or the downhole pump. A tubing drain valve would also be useful to inject chemicals into the formation or to aid in cleaning paraffin, wax and other deposits from the interior of the string.
The efficiency of production strings is often decreased by the deposit of paraffin, wax and similar substances on the interior of the string as the string rises through lower temperature zones from a deep, high-temperature production zone. It is necessary to remove these deposits in order to maintain the efficiency of the production string. Present methods for removing these deposits include flowing hot water or steam over the deposits. The hot water or steam is typically forced down the annulus between the production string and the borehole casing. The hot water or steam enters the production string through the downhole pump and returns to the surface through the string and the elevated fluid temperature slowly dissolves the deposits. Because the path to the zone of interest is long and circuitous, the cleaning fluid reaches the zone of interest at a temperature significantly lower than the injected fluid.
It is often desirable to inject the producing zone with a variety of chemicals to increase or aid production. These chemicals are injected by the previously described method for injecting steam or hot water. These methods suffer from the disadvantage that excessive quantities of fluids and chemicals are required because the injection is indirect through the annulus. Alternatively, dedicated small diameter injection tubing is employed to directly inject chemicals into the production zone.
It is often necessary to pull the production pump for routine maintenance, repair, replacement and the like. Many presently employed systems require that the production string be pulled in order to pull the production pump. This is necessary because the production pump will not pass through the interior of the production string due to size differential or obstructions in the string. Deep production strings may reach many thousands of feet or even several miles in length. The weight of these strings is quite substantial and significantly complicates the pulling of the string. Further, the weight of the fluid column within a long string adds many tons and contributes substantially to the weight complications. The equipment required to pull a "wet string," i.e., a string from which the production fluid has not been drained, must be capable of pulling this excessive and unnecessary weight. Accordingly, it is desirable to drain the fluid from the string prior to pulling the string or the downhole pump.
In an attempt to solve some of the above problems, others have proposed a variety of valves suitable for use in a borehole production string. For example, valves which open when the pressure within the tubing exceeds a predetermined pressure have been proposed. Such valves permit fluid communication between the interior of the production string and the annulus at the valve location. These valves are useful for draining the production string above the valve location and for injecting chemicals into the borehole at the valve location only if the valve remains open after its initial opening. It is typically necessary to pull the production string in order to close these valves. Alternatively, other check valves, e.g., spring loaded valves operable only above a predetermined pressure, permit injection of fluids into a borehole at the valve location but fail to provide a means for draining fluid from the production string.
Co-pending U.S. patent application Ser. No. 618,469 discloses a tubing drain valve opened when the pressure within the tubing exceeds a predetermined pressure and closed by a mechanical actuator located on the string of sucker rods for operating the downhole pump. However, that valve, like other valves actuated to the open position by application of a predetermined pressure to the fluid within the production string, typically does not perform satisfactorily when employed in strings for producing heavy, sand-bearing oils. These production strings are characterized and plagued by the precipitation and accumulation of sand and heavy paraffins in the production string. This accumulation often prevents the pressure applied to the production string from being applied to the tubing drain valve, necessitating the pulling of a "wet string."
Accordingly, there has been a long felt but unfulfilled need within the industry for a tubing drain valve which is useful in wells producing heavy, sand-bearing oils. This tubing drain valve might be conveniently and reliably actuated from the surface. This tubing drain valve might be actuated by commencement of withdrawal of the production string but must not be actuated by insertion of the string. This tubing drain valve must not be actuated by the partial withdrawal of the string during the addition and insertion of additional tubing joints during insertion of the string.